feat(lotus): in-frame screenshare button where the host can't delegate (cinny #43)

Firefox, Safari and WebKitGTK have no Capability Delegation, so the host
can't start a share from its call bar: getDisplayMedia needs the click in
this frame. Until now the host clicked EC's hidden footer button through
the DOM, which only works while the frame is same-origin.

With lotusHostControls + lotusFrameScreenshare the fork now shows EC's own
screenshare button (floating bottom-right, footer still hidden), so the
click is native. controls_state reports frameScreenshare so the host hides
its own button. New toWidget io.lotus.set_frame_screenshare { visible }
lets the host hide it while the room's call policy forbids screensharing
(never while a share is live, so it can still be stopped).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/useTheme.ts

@@ -83,9 +83,9 @@ export const useTheme = (): void => {
       // (The host used to inject `:root { color-scheme }` for this.)
       document.documentElement.style.colorScheme = theme;
     }
-    // [cinny #43] The host draws its own call bar: hide EC's footer, but keep
-    // it in the DOM (the host still clicks its screenshare button on engines
-    // without Capability Delegation). Replaces the host's injected styles.
+    // [cinny #43] The host draws its own call bar: hide EC's footer. Replaces
+    // the host's injected styles. Where the host can't start a share for us,
+    // LotusFrameScreenshare shows EC's screenshare button on its own.
     if (lotusFlag("lotusHostControls"))
       document.body.classList.add("lotus-host-controls");
   }, [previousTheme, requestedTheme]);